High-frequency transformer



April 11, 1950 w. A. M DONALD HIGH-FREQUENCY MSFORMER Filed June 16, 1943 INVENTOR. WILLIAM A. Mac DONALD 2&4. g &

ATTORNEY Patented A r. 11, 1950 2.503.879 mon-raaouancr TRANSFORMER William A. MacDonald, Locust Valley, N. Y., ll-

signor to Haaeltine Research, Inc Chicago. 111.,

a corporation of Illinois Application June 16, 1948, Serial No. 33,421

Claims. (Cl. 171-119) This invention relates to high-frequency transformers and. particularly, to tunable high-frequency transformers which are substantially enclosed in magnetic shields. Although the invention is of general application, it has particular utility in connection with tunable high-frequency band-pass selector systems such as those employed in the intermediate-frequency amplifierstages of a superheterodyne radio receiver.

Many radio receivers utilize a selector system including a tunable high-frequency transformer providing a pair of loosely coupled resonant circuits. Each such circuit includes one transformer winding with a fixed condenser coupled in shunt thereto and is tuned to resonance by adjustment of the inductance of the winding.

Individual core members, made of finely com-' minuted, high-permeability magnetic material bonded together by an insulating binder, are axially adjustable within individual ones of the windings for such inductance adjustments. Because of their high permeability and the positions thereof with respect to the windings, these core members provide a low reluctance path for the magnetic fiux of the windings, thus making it possible to realize with very small windings a relatively high factor of merit or "Q" for the resonant circuits. The Q of these resonant circuits may .further be improved by employing magnetic shields, made of the described magnetic material, which substantially enclose one or both of the windings. Incorporating such a shield into a high-frequency transformer of the type under consideration, however, often undesirably complicates the arrangement of the various components of the transformer and increases the manufacturing costs thereof out of proportion to the added benefits derived from the shield.

In the factory or in the service shop, it is usual to adjust the tunable high-frequency transformers of the type described to align their resonant circuits accurately to the intermediate frequency. During that stage in the manufacture of a radio receiver wherein final adjustments are being made, it is very desirable that the inductance adjustments of the various windings of the high-frequency transformers be made from but one side of the receiver chassis to reduce labor production costs. For periodic servicing operations, it frequently may also be important that all transformer adjusting points be accessible from one side of the receiver chassis. Consequently, it is desirable that each core member of a tunable high-frequency transformer '2 employingmagnetic. shields for the windings thereof be adjustable from one end of the transformer. Known high-frequency transformers having magnetic shields which substantially enclose the windings have not heretofore permitted this type of. adjustment.

It is an object of the invention, therefore, to provided a new and improved high-frequency transformer which is adjustable entirely from one end of the transformer to vary the inductance of the windings thereof.

It is another object of the invention to provide a new and improved high-frequency transformer which is simple in construction, easy to fabricate and assemble, and relatively inexpensive to manufacture.

It is a further object of the invention to provide a new and improved tunable high-frequency transformer which is compact in size, is characterized by a high Q, and is relatively easy to adjust for tuning purposes.

In accordance with a particular form of the invention, a high-frequency transformer comprises a pair of inductively coupled windings supported in coaxial and axially spaced relation. The transformer also includes a first member of magnetic material supported substantially to enclose at least one of the windings and including an outer portion and an apertured re-entrant portion having an axial length much shorter than that of the outer portion and axially movable relative to said one winding to adjust the self-inductance thereof. The transformer further includes a second member of magnetic material supported for axial movement relative to the other of the windings to adjust the self-inductance thereof. The adjustment of the second member is effected through the aperture in the re-entrant portion of the first member.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a longitudinal sectional view of a high-frequency transformer embodying a particular form of the invention; Fig. 2 is an exploded perspective view illustrating the assembly of several elements uti-' lized in the transformer of Fig. 1; and Fig. 3 is a schematic representation of the electrical circuits provided by the transformer of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, the high-frequency transformer there represented comprises a supporting means including a first portion or member i0 and a second portion or member I i. The member i0 is cup-shaped and is formed of a suitable dielectric material, for example a moldable thermosetting or thermoplastic resin. The member II, which may be of the same material, supports a pair of inductively coupled windings I2 and ii in coaxial and axially spaced relation. Its configuration is that of a circular disc and it is positioned coaxially within the cup-shaped member It. The member H has a pair of centrally disposed annular flanges It and ii on opposite sides thereof. A hollow cylindrical insulating coil form l8, which carries the winding l2, preferably fits tightly over the upper flange l6 and is secured thereto in any suit= able manner as by cementing. An insulating coil form l9 supports the winding l3 and is rigidly mounted within the recess in the lower flange H. The windings i2 and iii are so positioned on the coil forms l8 and I9 that they are loosely coupled.

The transformer also includes a cup-shaped member 23 of finely divided magnetic material in a suitable dielectric binder. The member-23 is supported by the member ID as shown, substantially to enclose the winding i2 and thereby provide a magnetic shield therefor. The member 23 is threaded into the cup-shaped first portion if) at its open end 25. The member 23 includes an integral annular re-entrant portion 2% extending into the winding i2. The member 23 and its reentrant portion 28 are axially movable relative to the winding l2, by rotation of the member 23, to adjust the self-inductance of this winding. The member 23 preferaby includes a hexagonal or other suitably shaped tool-receiving recess 29 to facilitate adjustment.

The transformer additionally includes a cyl-' indrical core member 38 of similar finely divided magnet c material which is supported by the member H for axial movement relative to the winding l3 to adiust the self-inductance thereof. In particular, the member 30 is secured to a threaded rod 3| of nonmagnetic material, such as brass. having a kerf 32 at the upper end thereof. The rod 3i is mounted in an axial threaded bore 33 of the flange l6, thus permitting axial adjustment of the core member 30 relative to the w nding l3 to be made by a screwdriver inserted through the aperture in member 23 and bro ght into engagement with the kerf 32. Preferably. the spacings between the windings l2 and I3 and the core members 28 and 36 in their norma positions are such that an adjustment of the self-inductance of either of the windings may be effected in the manner described without apprec ab v chan ing the self-inductance of the other winding while adjusting the mutual inductance between the windings approximately as the square root of the self-inductance variation to maintain substantially constant coefflcient of coupling and selectivity.

The high-frequency transformer further includes a cup-shaped core member 34 of magnetic material similar to that of the member 23. This member is supported coaxially within the member ID in a recess 35 at the base thereof and substantially encloses the winding l3 to provide a magnetic sh eld therefor. The member 34 also serves to maintain the second portion il in a predetermined snaced re ation with respect to the base of the member Ill.

The transformer also includes a cylindrical end cap 31 of insulating material having a diameter equal to that of the member It. This end cap has pairs of spaced apertures 88. 38 and I9. 39 (see also Fig. 2) extending therethrough and arranged in alignment with corresponding apertures in the base of the cup-shaped member i6. The apertures 38, 38 in the end cap 31 and the corresponding apertures in the member iii receive and position a pair of terminal elements 60, M of conductive strip material, illustrated more clearly in Fig. 2. The elements 60 have elongated body portions 4! and lug portions 42 bent toward each other and extending normaly to the body portions. The elements it, G0 are connected to. the winding i2 at their upper ends. A second pair of terminal elements 46, 66 (Fig. 2) extend through the other pair of apertures 39, 39 of the end cap 37 and-also through two similar apertures in the base of the member it. The terminal elements it, 88 have elongated body portions 8?, G1 which are connected to the winding iii. The terminal elements it, it also include lug portions 5!, 5i similar to the lug portions d2, :52 of the terminal elements (it, til. For some applications, it may also he desirabe to provide one of the terminal elements it and one of the terminal elements 66 each with a second lug portion 52, preferably a continuation of, and extending normall to, the lug portions 62 and Si, respectively. The end cap 3? is provided with a shallow recess 53 in the top face thereof and with an axial aperture 56.

For some applicat ons it may also be desirable to enclose the entire transformer in a conventional shield can 55 of conductive materla, such as alum num, which may be grounded. The shield can in ludes an aperture 56 at the upper end thereof to permit the insertion of an adjusting tool. The shie d can includes a pluralitv of dimples "it, It which are arranged to rest in corresnonding recess 7 i, ii in the end cap 3?, to secure the shield can in place.

In assembling the transformer described. one of each of the pairs of elements id, M3 and it, 38 are initially in erted in their corresponding apertures 38 and 3% with their respective lugs 62 and 5! lying fiat on the bottom of the recess 53. U on these is assembled a member 55 of insulating material. such as sheet mica. The latter member preferably has bonded to the upper and lower surfaces thereof. as by spraying with a metallic paint, conductive patches 5%, 5B individually having a confi uration corresponding to that of the corres onding lug portion ad acent thereto. The rema n ng termina s it and it are then inserted in their corres onding apertures 38 and 39 so that their ug ortions t2 and -l engage the conductive patches 56. '58 on the dielectric member 55. A rivet 59 is inserted through a central aperture $3 in the member 84. an aperture 69 in the member it. a erture 66 in dielectr c member 5%, and aperture 54 in the end cap 37 to fasten these members and the terminal elements together to form a unitary structure.

The coil forms is and i9, .wih the windings i2 and I3 thereon. and the adjustable core 3i) are assembled on the member H to form a unit.

This unit is then inserted in the member It so that the end portions of the terminal elements All, l0 and 4B. 46 slip through apertures 68. 68 in the disc. thus permitting the lower surface of the disc to abut the annular top surface of the shield member 34. If desired, the engagement between the member ii and the inner surface of the member in may be by way of a press fit. The upper ends of the terminal elements 40, 40 and ll, 45 are connected to their respective coil terminal and are then bent firmly down against the upper surface of the disc II, as illustrated in Fig. 1. Accordingly, the terminal elements 40, 40 ani 45, 45 with their several lug portions are effective in a very simple and convenient manner rigidly to secure the member In to the member Ii to form a unitary structure. The member 23 is threaded into the top of the member ID and the shield can 65 is placed over the entire structure.

Considering now certain of the electrical features of the transformer, the dielectric member 55, when assembled in the position represented in Fig. 1, forms with the terminals l0, 40 a condenser 50 (shown schematically in Fig. 3) having a'fixed capacitance which is in parallel with the winding l2. It also forms with the terminals ll, 45 a condenser 6| (Fig. 3) having a fixed capacitance in parallel with winding i3, Likewise, the dielectric member 55 forms with the lu portions 52, 52 a third condenser 62 (Fig. 3) providing a fixed capacitive coupling between the windings I2 and I3.

Depending upon the particular terminal connections of the latter to the terminal elements Ill and 45 associated with the condenser 52, the latter may either aid or oppose the mutual inductive coupling between the windings. The use of the condenser 62 thus may aid in the attainment of a relatively sharp or a relatively broad selectivity characteristic as desired for a particular application. It will be understood, of course, that the condenser 62 may be omitted entirely from the transformer construction by omitting the lugs 52, 52 and the corresponding portion of the patch 56 of the member 55.

It will be apparent from the foregoing description that, since the windings l2 and I3 are substantially enclosed in a magnetic shield of finely divided magnetic material, the transformer windings may be small and the tuned circuits of the transformer will have relatively low losses and, hence, a high Q. Consequently, a high gain and a stable band-pass frequency characteristic may be procured from receiver stages employing such a transformer. For those applications wherein it is desirable to employ a shield can 55 for the transformer, the losses in the shield can are minimized because of the shielding action afforded by the magnetic core members 23 and 34. It will further be apparent from the foregoing description that a tunable high-frequency transformer embodying the invention is relatively easy to adjust since both adjustments of the selfinductance of the transformer windings may be made from one end of the transformer. It will also be clear that a high-frequency transformer embodyin the invention is simple in construction, easy to fabricate and to assemble, and relatively inexpensive to manufacture.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A high-frequency transformer comprising: a pair of inductively coupled windings supported in coaxial and axially spaced relation; a first member of magnetic material supported substantially to enclose at least one of said windings and including an outer portion and an apertured 6 re-entran't portion having an axial length much shorter than that of said outer portion and axially movable relative to said one winding to adjust the self-inductance thereof; and a second member of magnetic material supported for axial movement relative to the other of said windings to'adjust the self-inductance thereof, said adjustment of said second member being effected through the aperture in said re-entrant portion of said first member.

2. A high-frequency transformer comprising: a pair of inductively coupled windings; means including a cup-shaped first portion and including a second portion positioned within said first portion for supporting said windings in coaxial and axially spaced relation; a rotatable first member of magnetic material supported in threaded engagement with said first portion at the open end thereof and substantially enclosing one of said windings and including an outer portion and an apertured re-entrant portion having an axial length much shorter than that of said outer portion and axially movable relative to said one winding by the rotation of said first member to adjust the self-inductance of said one winding; a second member of magnetic material supported by said second portion for axial movement relative to the other of said windings to adjust the self-inductance thereof, said adjustment of said second member being effected through the aperture in said re-entrant portion of said first member; and a third member of magnetic material supported within said cup-shaped first portion at the base thereof and intermediate said base and said second portion substantially to enclose said other of said windings to provide a magnetic shield therefor.

3. A high-frequency transformer comprising: a pair of inductively coupled windings supported in coaxial and axially spaced relation; a first member of magnetic material supported substantially to enclose at least one of said windings and including an outer portion and an apertured re-entrant portion having an axial length much shorter than that of said outer portion and axially movable relative to said one winding to adjust the self-inductance thereof independently of and without appreciably changing the self-inductance of the other of said windings; and a second member of magnetic material supported for axial movement relative to said other winding to adjust the self-inductance thereof independently of and without appreciably changing the self-inductance of said one winding, said adjustment of said second member being effected through the aperture in said re-entrant portion of said first member.

4. A high-frequency transformer comprising: a pair of inductively coupled windings; means including first and second portions for supporting said windings in coaxial and axially spaced relation; 9. first member of magnetic material supported by one of said portions substantially to enclose one of said windings and including an outer portion and an apertured re-entrant portion having an axial length much shorter than that of said outer portion and axially movable relative to said one winding to adjust the selfinductance thereof; a second member of magnetic material supported by said second portion for axial movement relative to the other of said windings to adjust the self-inductance thereof, said adjustment of said second member being effected through the aperture in said re-entrant portion of said first member; and a third mem- 7 her of magnetic material supported between said first and said second portions substantially to enclose said other of said windings.

5. A high-irequency transformer comprising: a. pair of inductively coupled windings; means including first and second portions for supporting said windings in coaxial and axially spaced relation; a first member of magnetic material supported by one of said portions of said means sub= stantially to enclose one of said windings and including an outer portion and an apertured re entrant portion having an axial length much shorter than thatof said outer portion and axially movable relative to said one winding to adjust the self-inductance thereof; a second member of magnetic material supported by said second portion for axial movement relative to the other ofaaid windings to adjust the self-inductance thereof, said adjustment of said second member being efiected through the aperture in said re-entrant 20 portion of said first member; a first pair of terminal elements coupled to'said one winding; and

d a second pair of terminal elements coupled to said other winding; said first and said second pair oi terminal elements extending between said first and said second portions of said means and being so arranged as to maintain said mst mil-tion thereof in a unitary assembled relation with reference to said second portion thereof.

W'IILIAM A. MACDONALD.

REFERENQES (DETED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,348,584 Jacob Apr. ll, 19% 2,430,990 Moore Nov. 18, 194? 2,435,630 Ketcham Feb. 10, 1945 FOREIGN PATENTS Number Country Date 457,263 Great Britain Nov. 24, 1936 

